Targeted drug delivery has the potential to improve the efficacy of a therapeutic agent while reducing its side effects. Folate receptor type-beta (FR-beta) is a cell surface marker selectively expressed by approximately 70% of acute myeloid leukemias (AMLs). Increased FR-beta expression can be specifically induced by all-trans retinoic acid (ATRA) in primary AML cells and in FR-b (+) KG-1 cells, without inducing cellular differentiation or growth inhibition. Folic acid is a high affinity ligand for FR-beta (Kd < 1 nM). Importantly, FR-beta expressed by normal hematopoietic cells cannot bind folate in contrast to that in primary AML cells, KG- 1 cells, and FR-beta-transfected CHO cells, all of which mediate selective uptake and cytotoxicity of folate-coated liposomal doxorubicin (f-L-DOX). FR-beta-targeted uptake and cytotoxicity of f-L-DOX were further enhanced by inducing FR-beta upregulation using ATRA. F-L-DOX also exhibited greater therapeutic efficacy than non-targeted liposomal DOX (L-DOX) in FR (+) murine L1210JF and human KG-1 AML ascitic tumor models. Moreover, ATRA treatment further increased survival in response to treatment with f-L-DOX in the KG-1 cell engrafted SCID mice. FR-targeted liposomal DOX delivery has also been shown to bypass P-glycoprotein-mediated drug efflux in FR (+) tumor cells exhibiting resistance to free DOX. The objective of this Phase I project is to extend and further establish the value of this type of selective targeting using a related but potentially superior anthracycline drug, daunorubicin (DNR) and the superior NOD/SCID engraftment model. F-L-DNR combined with ATRA, will be evaluated as a therapy for AML using an animal model that more closely mimics human leukemia. The Specific Aims are: 1) to extend a human AML murine NOD/SCID engraftment model to different AML subtypes; 2) to evaluate the effect of ATRA on FR-beta expression by AML cells in the NOD/SCID model; 3) to evaluate the therapeutic efficacy of f-L-DNR, alone or combined with ATRA, in the NOD/SCID model. The data will be used to develop a plan for clinical studies of f-L-DNR/ATRA therapy in a Phase II project.